The invention relates to a mulching paper degradation process as well as to the mulching paper for implementing the process. As will be described below, the invention also relates to an enzymatic solution for implementing the said process.
In the rest of the description, the degradation process and the mulching paper of the invention are more particularly described in relation to the mulching of lettuces.
However, the paper applies to the mulching of all types of plants, such as strawberry plants, vines, tomatoes, courgettes, aubergines, peppers, cucumbers, shallots, etc.
Several mulching materials have been proposed.
Firstly, black plastic sheeting is known which has the advantage of being strong, despite its low weight per unit area, and thus being weather-resistant. However, such plastic sheeting has a certain number of drawbacks, especially that of not being biodegradable. For obvious environmental reasons, plastic cannot be buried in the soil. Consequently, when the harvesting has been completed and the grower wishes to plough his land, he must completely remove the plastic. The cost of this operation is estimated to be 20% of the total cost of the mulching.
Moreover, since the plastic heats up very quickly when exposed to solar rays, the lettuce leaves close to the ground adhere and remain adhered to the mulching film and therefore can only be removed with removal of this film. It is estimated that the removal of one ton of plastic entails the removal of one ton of plants and earth that have remained attached to the said plastic. Consequently, the plastic is virtually unrecyclable, the cost of washing it being unacceptably high for such an operation.
To solve these various problems, mulching papers have been developed at the same time. Such papers, more particularly described in documents FR-A-2,016,071, are treated with urea-formaldehyde resins so as to increase their wet strength and thus decrease the risk of tearing when used on the ground.
However, despite this treatment, the paper can break due to the effect of the watering phases, which expand the paper, and of the drying phases which retighten the paper.
Moreover, and above all, the treatment of the paper with hydrophobic resins inevitably results in its biodegradability slowing down. It has in fact been found that complete biodegradation of these types of paper is achieved only after 2 years.
In other words, one is faced with the complex problem of using a mulching paper which is weather resistant over a defined period but which at the same time can be degraded in a minimum time after harvesting.
To solve this problem, the Applicant has developed a process for the accelerated degradation of mulching paper whose cellulose fibres are impregnated with a hydrophobic resin giving the said paper a weather-resistant character, in which an enzymatic solution capable of degrading both the resin and the cellulose is sprayed on to the paper.
In other words, the process of the invention consists in depositing over the growing area a paper whose fibres are impregnated with a resin providing it with wet strength, and therefore making it non-tearable, while being compatible with the period of use. Thus with regard to strawberry plants, for example, the paper must be kept in place over the growing area for 9 months, whereas this period is only 6 weeks in the case of lettuces. After harvesting, the paper is degraded by spraying an enzymatic solution, the said solution allowing not only the rate of degradation of the cellulose to be considerably accelerated but also preventing any accumulation of resin in the soil by degradation of the resin. To further accelerate the degradation of the paper, the ground is then ploughed so as to bury the paper in the soil.
According to a first characteristic of the process of the invention, the hydrophobic resin is chosen from the group comprising urea-formaldehyde resins, melamine-formaldehyde resins, polyamide-amine-epichlorohydrin resins, polyethyleneimine resins and starch derivatives, by themselves or as a mixture.
These resins are known to those skilled in the art and are specifically described in document xe2x80x9cle point sur . . . xe2x80x9d, No. 11, January 1986 from Centre Technique du Papier de Grenoble [Grenoble Paper Technical Centre].
In one advantageous embodiment, the chosen resins are polyamide-amine-epichlorohydrin resins.
Moreover, in order to degrade not only the resin but also the cellulose, the enzymatic solution comprises enzymes capable of catalyzing the hydrolysis of resin-resin, resin-cellulose and cellulose-cellulose bonds.
In the rest of the description and in the claims, the expression xe2x80x9cresin-resin bondsxe2x80x9d should be understood to mean the hydrolyzable bonds between monomers of the resin.
Likewise, the expression xe2x80x9cresin-cellulose bondsxe2x80x9d should be understood to mean the ester bonds linking the resin to the cellulose.
Finally, the expression xe2x80x9ccellulose-cellulose bondsxe2x80x9d should be understood to mean the glycosidic bonds of the cellulose.
To catalyze the hydrolysis of the ester bonds linking the resin to the cellulose, the enzyme chosen will be an esterase.
Likewise, to catalyze the hydrolysis of the glycosidic bonds of the cellulose, the enzyme will be a cellulase chosen from the group comprising exocellobiohydrolases, endoglucanases and xcex2-glucosidases, alone or as a mixture.
Finally, the choice of enzyme capable of catalyzing the hydrolysis of the bonds between monomers forming the resin will be chosen depending on the nature of the said bonds.
In practice, with regard to polyamide-amine-epichlorohydrin resins, the enzyme will be chosen from the group comprising amidases and aminases.
As already stated, the invention also relates to the mulching paper for implementing the process described above.
Apart from the fact that the mulching paper of the invention must have a composition such that it can be degraded according to the process described above, it must also have a certain number of properties allowing it to be used as mulching material.
Thus, the paper must be resistant to the weather, such as rain and wind, and to extreme temperature conditions, such as cold and hot conditions, and consequently must have excellent mechanical properties.
Consequently, the Applicant was set the objective of obtaining a paper having a bursting strength of between 150 and 250 kPa.
Another characteristic that the paper must have is that of being permeable to water in so far as the spraying water must be able to irrigate the soil and the roots of the plants. However, the pores in the paper must have a volume such that they limit the evaporation of water lying beneath the mulching so as to preserve""sufficient moisture in the soil.
To obtain such properties, the Applicant set the objective of obtaining a paper having an air permeability of between 1500 and 3500 cm3 over an area of 10 cm2 under a vacuum of 1 kPa in sixty seconds.
To achieve all these objectives, the mulching paper of the invention is characterized in that it comprises a fibrous composition containing, by weight:
from 40 to 60%, advantageously 50%, coniferous unbleached kraft paper pulp;
from 10 to 30%, advantageously 20%, deciduous unbleached kraft paper pulp;
from 20 to 40%, advantageously 30%, deinked packaging pulps.
The unbleached kraft paper pulps and their method of manufacture are well known to those skilled in the art. Likewise, the deinked packaging pulps are known as resulting from packaging residues.
According to another characteristic, the mulching paper furthermore contains from 0.5 to 15% resin, advantageously from 7 to 8%, resin by weight with respect to the weight of the paper.
For a resin concentration of less than 0.5% by weight, the paper does not have a sufficiently high wet strength and it tears during spraying or when it rains. For a resin concentration of greater than 15% by weight, a paper having optimized mechanical properties is not obtained, so that the paper becomes economically less advantageous.
Moreover, to avoid the development of microorganisms on the surface of the paper, the paper furthermore contains a fungistatic agent.
In one advantageous embodiment, the fungistatic agent is distributed only over those areas of the paper sheet which are intended to be buried. In this case, the fungistatic agent is deposited at the wet end of the paper machine by spraying it using air-pressurized nozzles.
In addition, in order to increase the extensibility of the paper when the crops are sprayed, while still preventing it from breaking when drying out, the paper is in creped form. This pleating obtained by a creping operation on the paper machine furthermore makes it possible to adapt the paper, on the one hand, to the contour of the ground by accepting the slight curvatures in the latter and, on the other hand, to its dimensional variations due to the action of the successive spraying and drying phases.
The invention also relates to the enzymatic solution for implementing the accelerated mulching paper degradation process described above.
Of course, and as has been already stated, the composition of the said enzymatic solution will vary depending on the resin used and on the nature of the sites at which the said enzymes must act.
However, and in practice, the solution always contains cellulases capable of breaking the cellulose-cellulose bonds and esterases capable of breaking the resin-cellulose bonds.
According to an advantageous embodiment, the enzymatic solution contains, by weight:
from 10 to 20% cellulase;
from 15 to 40% amidase;
from 15 to 406 esterase;
from 10 to 35% aminase.
In practice, the enzymatic solution is sprayed in an amount of 0.01 to 1 g, advantageously 0.3 to 0.4 g, of pure enzyme per m2.
In another embodiment of the invention, the enzymatic solution may be incorporated directly into the paper, within microcapsules.
Consequently, the mulching paper will be degraded after harvesting, after being buried in the soil, especially by ploughing, that is to say when the walls of the microcapsules containing the enzymatic solution have broken due to the mechanical action.
In other words, the invention also relates to a mulching paper capable of rapidly degrading after being buried in the soil, the cellulose fibres of which are impregnated with a hydrophobic resin giving the said paper a weather-resistant character, characterized in that it furthermore comprises microcapsules filled with an enzymatic solution, the walls of which microcapsules are broken during burying in the soil, thus releasing the enzymes capable of catalyzing both the hydrolysis of the resin and that of the cellulose.
The manufacture of the enzymatic solution microcapsules and their incorporation into the paper constitute known operations within the scope of those skilled in the art and will not be described below any further.
Moreover, and as previously, the enzymatic solution contains enzymes capable of catalyzing the hydrolysis of the resin-resin, resin-cellulose and cellulose-cellulose bonds.
Advantageously, the enzymes are chosen from the group comprising esterases, cellulases, amidases and aminases.
In order for the paper to be sufficiently weather resistant, it comprises from 0.5 to 15%, advantageously from 7 to 8%, by weight of resin with respect to the weight of the paper.